This invention relates to the field of wide angle narrowband optical filters.
Spectral filters are employed for selecting a portion of the spectral content of an incoming broadband optical signal. Many optical systems require a spectral filter having a very narrow spectral passband width but a very large acceptance angle for incident light. A narrowband optical filter is an essential element, for example, in many laser communications systems to reject unwanted background light and thereby increase the signal-to-noise ratio. In addition, the signal in such systems may be transmitted through a random medium, such as the earth's atmosphere or sea water, in such a way that what must be observed is a scattered component of the signal. The signal may thus appear to come from a wide field of view of up to several steradians. A particular example of such circumstances may be found in submarine communications systems, where a filter with a bandwidth of less than 1 .ANG. and an acceptance angle of at least .+-.30.degree. is required.
Unfortunately, in the spectral filters known in the art, a narrow bandpass width is invariably accompanied by a narrow angular aperture. The angular aperture of a narrowband dielectric interference filter, for example, is limited by the relationship EQU .theta..sup.2 =2N.sup.2.sub.eff (.DELTA..lambda./.lambda.), (1)
where .theta. is the acceptance cone angle, (.DELTA..lambda./.lambda.) is the fractional bandwidth, and the quantity N.sub.eff is the effective index of refraction of the interference filter. As can be seen from Equation 1, a dielectric interference filter with a fractional bandwidth .DELTA..lambda./.lambda. of 10.sup.-4 and an effective index N.sub.eff of 2.0 will be limited to an angular aperture .theta. of approximately 1.5.degree.. This same relationship between the angular aperture and the fractional bandwidth also holds for polarization interference filters, such as Solc filters and Lyot-Ohman filters. Some improvement in the angular aperture of the Lyot-Ohman filter can be obtained by replacing the birefringent plates with more complicated birefringent plate combinations, raising the angular aperture for a quartz Lyot filter with a fractional bandwidth of 10.sup.-4 to approximately 16.degree.. Unfortunately, however, polarization interference filters are disadvantaged because an inherent signal loss of 50% occurs in the front polarizer of such a filter.
As a consequence of these limitations in the prior art, a need has developed for a low loss spectral filter which will exhibit a very narrow bandwidth and a wide angular aperture.